Broadcast transmissions often include ancillary signals such as background music or reading services for the blind along with a main carrier signal. The Radio Data System (“RDS”) standard is an international standard that defines a basic FM subcarrier digital coding and transmission system. The most current and widely used data transmission standard in the United States is the Radio Broadcast Data Systems (“RBDS”) standard. The RBDS standard defines how to implement the RDS standard in North America.
The RBDS standard, published by the National Radio Systems Committee and sponsored by the Electronic Industries Association and the National Association of Broadcasters, describes a system for broadcasting a variety of program-related information on a subcarrier of a standard FM broadcast channel. It was designed to allow stations to send information such as call letters, station format, traffic alerts and text messages to compatible radios.
RDS encoders generate what is known as a “subcarrier” that is transmitted along with an FM station broadcast signal and can be demodulated by special decoders. The RDS uses a subcarrier frequency of 57 khz. Commercially available RDS encoders usually accept information via either serial or parallel data ports and format the information into the appropriate RDS block type.
The RDS data signal is a specially encoded data stream containing up to 32 repeating “groups” and is transmitted at 1187.5 bits/second. The RDS data signal does not require inclusion of all potential data group blocks of both repeating and unique data. Certain embodiments includes using one of several groups that are designed for data transmission functions.
An RDS data group is composed of 4 blocks, each divided by checkwords used for error correction. Block 1 is a 16-bit Program Identification code (PI) which contains a country symbol, a regional code, and a number which is derived from the transmitting station's call letters. Block 2 includes a 4-bit group type code and a 1-bit group version code that identifies the type of information the data group contains. This block also contains a 1-bit code that identifies the transmitting station as one that broadcasts traffic information, followed by a 5-bit Program Type (PTY) code which describes the current program or format being broadcast by the station (Rock, Oldies, Talk, News, etc.).
Information contained in Blocks 3 and 4 are dependent on the codes included in Block 2. Blocks 3 and 4 provide two 16-bit data slots where specific information can be sent to the special receiver. For example, RDS Group type 2A uses blocks 3 and 4 to transmit a segmented 64-character text message known as RadioText (RT). This appears on RDS-enabled radios as a message that some stations use to identify the song or program. Other group types use these blocks to identify alternate frequencies where the same programming can be available, in-house station text messages, or Emergency Alert System (EAS) communication messages. An extensive description of the RDS standard is available through the National Association of Broadcasters and the National Radio Systems Committee.
Broadcasters using the RDS standard can distribute information to a large number of users. However, the standard does not provide for individual users to respond to the broadcast information.